Has the account holder spoken to Sky about this so that the phone can be blocked as per this:
https://www.sky.com/help/articles/lost-or-stolen-sky-mobile
And you might want to register the stolen device here:
https://www.immobilise.com/
He might be able to track it depending on what he had via either of these:
https://support.apple.com/en-gb/HT201472
https://support.google.com/accounts/answer/7177579?
When the call to Sky is made as them for a replacement sim which will come with the same number as the lost one.
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